Spectroscopic measurement of sub-Doppler cooling with two color σ+σ- laser configuration

Abstract

We investigated the temperature and the position of the atomic cloud in a magneto-optical trap (MOT) of 85Rb atoms under a non-zero magnetic field. Toward this end, laser detunings were set differently for the counter-propagating laser beams so that the atoms were trapped where the magnetic field was nonzero. We found that the sub-Doppler cooling effect, known to decrease in an MOT if the magnetic field increases, would in fact increase under a specific condition. This condition was first set forth by theoretical considerations and then was verified experimentally by conducting non-destructive measurements of atomic temperature based on photon-counting heterodyne spectroscopy of the resonance uorescence of trapped atoms. A simple way to load atoms in a non-zero magnetic field is to apply different laser detuning frequencies for the counter-propagating trap lasers in an MOT. In this case, the optical pressure of each counter-propagating laser is different. The optical pressure difference induces the Doppler cooling, and the MOT loads atoms at a position with the non-zero magnetic field that can compensate the detuning difference. Note this position is generally different from the resonant point of the sub-Doppler cooling. Therefore, as the laser detuning increases, both the temperature of the atoms and the magnitude of the magnetic field at the center of the atomic cloud increases. Recently, we have conducted a research on the interaction between the trapped atoms and an optical lattice in a passively stabilized MOT. The optical lattice was made in the MOT by stabilizing the relative phases of the trap laser beams. More recently, we have investigated the effect of a moving optical lattice. We found that the atoms localized at the potential minima in a stationary lattice are no longer localized due to the tunneling through the lattice potential walls when the optical lattice was made to move rapidly. In the present study, we investigated the temperature and the position of the atomic cloud in the σ+-σ- laser configuration with different frequency detunings. From the experimental result and its analysis, we then obtained the condition for maximizing the sub-Doppler cooling under a non-zero magnetic field. In addition, we observed the suppression of heating, caused by the atomic oscillation in the moving optical lattice. These results were obtained by non-destructively measuring the temperature of the thermal gas.